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BELL SYSTEM TECHNICAL JOURNAL 



lower core losses. Another important difference between the open 

 wire and cable coils was the use of high dielectric strength insulation 

 in the open wire coils. The coils were subjected to a breakdown test 

 at 8,000 volts (effective a-c.) and were protected in service by means 

 of a special type of lightning arrester having non-arcing metal elec- 

 trodes designed to operate at 3,500 volts direct current. 



Table IV lists the principal characteristics of the loading coils 



TABLE IV 



First Standard Loading Coils 



Note. Effective resistance values apply for a line current of . 002 ampere. 



initially used in the standard loading systems listed in Tables II 

 and III. 



Loading Coil Cases. The cases used for potting the cable loading 

 coils w^ere designed so that they could be installed in underground 

 manholes or on pole fixtures. 



The general method of assembly is to dry the loading coils thor- 

 oughly and then impregnate them under vacuum with a moisture- 

 proofing compound. The coils are then mounted on wooden spindles, 

 adjacent coils being separated by iron washers. After carefully ad- 

 justing the individual coil« to meet the electrical requirements, the 

 spindles of coils are cabled to a short length of lead-covered cable 

 which is referred to as a "stub" cable. Cast-iron cases with iron 

 l)artitions were designed vSO as to provide a shielded compartment 

 for each spindle of coils. 



CommercialK- manufactured toroidal coils may have small irregu- 

 larities in their windings resulting in a weak stray field which tends 

 to cause cross-talk. The iron washers between coils and the parti- 

 tions between spindle groups of coils provide effective cross-talk 

 shields. 



